Microscopic theory of angular momentum distributions across the full range of fission fragments

TL;DR

This paper develops a microscopic theoretical framework to predict angular momentum distributions of primary fission fragments, providing detailed insights that improve the accuracy of nuclear fission models.

Contribution

It introduces a novel combination of projection techniques and configuration mixing to calculate angular momentum distributions for all fragments in neutron-induced fission, a first in the field.

Findings

Revealed a sawtooth pattern in average angular momentum across fragment masses.

Found significant variations in angular momentum distributions along isobaric chains.

Identified correlations between angular momentum and fragment deformation at scission.

Abstract

Modern nuclear theory provides qualitative insights into the fundamental mechanisms of nuclear fission and is increasingly capable of making reliable quantitative predictions. Most quantities of interest pertain to the primary fission fragments, whose subsequent decay is typically modeled using statistical reaction models. Consequently, a key objective of fission theory is to inform these models by predicting the initial conditions of the primary fragments. In this work, we employ a framework that combines joint angular momentum and particle number projection with time-dependent configuration mixing to calculate the angular momentum distributions of primary fragments. Focusing on the benchmark cases of neutron-induced fission of $^{235}$U and $^{239}$Pu, we predict - for the first time - microscopic angular momentum distributions for all fragments observed in experiments. Our results reveal a pronounced sawtooth pattern in the average angular momentum as a function of fragment mass, consistent with recent measurements. Additionally, we observe substantial variations in angular momentum distributions along isobaric chains, indicating that commonly used empirical formulas lack sufficient accuracy. We also quantify a strong correlation between the angular momentum and the deformation of the fragments at scission, and a weak correlation in the magnitude of the angular momentum between fragment partners. The generated data will enable estimation of the impact of microscopic distributions on fission spectra, paving the way toward fission modeling based on microscopic inputs.